Method for production of a DFB laser diode having a coupled optical waveguide and a DFB laser diode layer structure.
The invention relates to a method for production of a MCRW-DFB laser diode with a coupled optical waveguide and a MCRW-DFB laser diode layer structure.
Optical data transmission has now gained considerable importance in long-distance traffic, but it is still not very widespread in the subscriber connection area, for cost reasons. In order to give the network subscriber access to distributive and interactive services and to the corresponding unidirectional and bidirectional data streams via his connection, the connection modules used here contain as primary functions the transmitter in the form of a semiconductor laser and the receiver in the form of a photodiode. A monitoring diode for monitoring the transmitting laser and wavelength-selective filter are added thereto as a secondary function in order to suppress crosstalk by separation of the different transmitted and received wavelengths. In order to service the demand for large quantities, which can be expected on the basis of system aspects, these modules must be produced cost-effectively.
Commercial transmitting/receiving modules having the four described functions of a laser diode, monitoring diode, receiver diode and filter are produced using microoptic construction techniques in accordance with the current prior art (see the publication "BIDI Bidirectional Modules" No. B155-H6656-X-X-7400 (1993) from Siemens AG). In this case, the large number of individual parts leads to relatively labour-cost-intensive assembly and adjustment steps. In the context of this technology, there are a number of options for further cost reduction, for example by using prefabricated subunits, for example a fibre-lens unit, a laser-monitor unit, inter alia, by development of a more economic construction technique while increasing quality, yield and life of the discrete module components. An example of this is the transition from the MCRW laser (MCRW stands for metal clad ridge waveguide) having a homogeneous laser-active layer to the MCRW laser having a strained quantum well structure (SL-MQW structure, where SL-MQW stands for strained layer multiple quantum well). In addition to the known advantages of the MCRW structure, which are a single epitaxial process, low leakage current and long life, this laser additionally offers a low threshold and a high maximum operating temperature, so that a high-temperature laser is produced. The characteristics of an MCRW laser for 1.3 .mu.m are described in detail in B. Stegmuller, E. Veuhoff, J. Rieger and H. Hedrich "High-temperature (130.degree. C.) CW operation of 1.53 .mu.m InGaAsP ridge-waveguide lasers using strained quaternary quantum wells", Electronics Letter, Vol. 29 (1993), No. 19, pages 1691-1693.
In the relatively old German Patent Application 20 P 44 04 756.8, the monolithic integration of the four functions on a substrate made of semiconductor material, for example InP, is proposed. Planar technologies are used for this purpose, as have been proven for the production of silicon chips. The photonic integration on InP in this case, in detail, makes use of technologies which are now largely prior art for discrete InP components. This includes epitaxial processes, lithography, etching techniques etc. Adjustment processes between microoptic parts have been replaced by photolithographically defined component structures. Standardization of the technology is leading to identical, interchangeable installations and process steps. The number of parts in the module, which is reduced by monolithic integration, reduces assembly times and improves the robustness. A method for production of a DFB laser diode (DFB stands for distributed feedback) and a DFB laser diode layer structure produced in accordance with this method are proposed in specific form in this document.
European reference-A-0 454 902 discloses a monolithically integrated circuit that comprises, among other things, a MCRW-DFB laser diode structure with butt-coupled waveguide and also discloses a 4-step epitaxy method for the manufacture thereof. The laser diode comprises a solid quaternary active layer. The DFB grating structure is arranged under the active layer.
A compressively strained MQW-DFB laser diode having a monolithically integrated waveguide, which is continuous, as well as a two-layer grating structure arranged above the active layer proceeds from Electronics Letters 28 (1992), p. 2361.